Fused Disco Required for HVAC?

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JFletcher

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Williamsburg, VA
Ran into a 40A fused disco (x2) on a residence last week. The insides are unbelievably corroded, so I recommended replacement, tho I would go with a standard (non fused) outdoor disco. The 2 units are wired on separate 40A breakers. Is there any code requirement for a fused disconnect here?

Also trying to figure out how a good outdoor enclosure is so corroded. The covers were on properly and the outsides look good. The insides look like they were sprayed with bleach and salt water. All I can figure is rainwater got in by the wiring...
 
You would need to comply with the manufacturer's nameplate per Art 440.22(C). If the requirement is for "fuses" then fuses need to be used in the protection.
If the existing 40 amp breakers satisfy the nameplate requirement then you can use a NF disconnect.
 
I like to use a fused disconnect when I am replacing condensors where the breaker at the main panel is, for instance, 50amp, but the nameplate calls for MOCP to be 40amp. The circuit wiring is still protected as it should be, and the condensor is also protected as it should be. I don't see the need for a disconnect with a breaker unless the manufacturer calls for one.
 
JFletcher said:
Also trying to figure out how a good outdoor enclosure is so corroded. The covers were on properly and the outsides look good. The insides look like they were sprayed with bleach and salt water. All I can figure is rainwater got in by the wiring...
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Is it possible that a service technician was sloppy when cleaning the condensor coil, and had over spray enter the open disconnect cover?
 
David Goodman said:
I like to use a fused disconnect when I am replacing condensors where the breaker at the main panel is, for instance, 50amp, but the nameplate calls for MOCP to be 40amp. The circuit wiring is still protected as it should be, and the condensor is also protected as it should be. I don't see the need for a disconnect with a breaker unless the manufacturer calls for one.
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For most resi applications unless it specs fuses, I prefer a NF disco, no fuses or breaker in disco. No need IMO.

Why would you have a 50A breaker on a unit that calls for a 40A MOCPD and then install 40A fuses? Just use a 40A breaker, unless speced fuses, and a NF disco, a lot easier and simpler IMO.
 
jumper said:
... Why would you have a 50A breaker on a unit that calls for a 40A MOCPD and then install 40A fuses? ...
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Just a guess, but perhaps no cut sheet and all wired up to disco before condenser unit was on site... :angel:
 
David Goodman said:
Is it possible that a service technician was sloppy when cleaning the condensor coil, and had over spray enter the open disconnect cover?
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Maybe, I dunno. Now that you mention it, I have seen PTACs with just trashed (badly corroded) aluminum fins on the coils. I've used many a can of coil cleaner and never seen them eat metal like that.

I originally checked them due to an "odd smell", which I didnt smell, but I remember finding a burned up disco once causing a dead animal like smell.

I agree with Jumper; I'd change out the 50A for a 40A at the panel, unless it was something I'd rather not get into w/o demoing it.

They are Trane units, so I looked up "do Trane require fused disconnects" and got this thread here:

http://forums.mikeholt.com/showthread.php?t=144721

So, if the unit calls for 40A fuses, do I still need fuses, or is a 40A breaker fine? I understand 110.3(B), but not the mentioned UL change in the above linked thread, which would seem to supersede 110.3(B) in this case (see post #8)

The HO still has the instruction manual for these units, which are much newer than the discos. I'll check the nameplates as well. For all I know the fuses and the breaker could both be wrong, especially since the units have been changed out
 
jumper said:
Why would you have a 50A breaker on a unit that calls for a 40A MOCPD and then install 40A fuses? Just use a 40A breaker, unless speced fuses, and a NF disco, a lot easier and simpler IMO.
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He's talking about replacing an old unit with a new one. The old unit had a 50 amp circuit now he needs a 40 amp circuit. I would guess that it's a toss up about which is easier to replace the CB or fuses.
 
jumper said:
For most resi applications unless it specs fuses, I prefer a NF disco, no fuses or breaker in disco. No need IMO.

Why would you have a 50A breaker on a unit that calls for a 40A MOCPD and then install 40A fuses? Just use a 40A breaker, unless speced fuses, and a NF disco, a lot easier and simpler IMO.
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My comment was concerning replacing old condensors with higher efficiency condensors, or as is very common, the original installation was over sized by the builder (bigger must be better). The unit being pulled out is what the home was spec'd. out for with a 50 amp. breaker and appropriate wiring. The new more efficient unit calls for 40 amp. MOCP. I don't see any need to reduce the breaker at the main to 40 amp.. Just add the appropriate MOCPD as part of the required disconnect.
 
JFletcher said:
The HO still has the instruction manual for these units, which are much newer than the discos. I'll check the nameplates as well. For all I know the fuses and the breaker could both be wrong, especially since the units have been changed out
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and they are. MOCP/Max fuse size is 25A (breakers or fuses allowed per nameplate). Dumping the corroded fused discos for unfused, changing the QO 40A breakers to 25A.
 
infinity said:
He's talking about replacing an old unit with a new one. The old unit had a 50 amp circuit now he needs a 40 amp circuit. I would guess that it's a toss up about which is easier to replace the CB or fuses.
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Fuses would be if the discos werent trashed. Since I've got to replace the disconnects, I can save the customer money and have a simpler (one OCPD) system by doing unfused discos and the proper breaker.

If it were a FPE panel or something similarly obsolete/problematic, I'd probably just get new discos with 25A fuses and not touch the panel. By doing this on a weekend, I'm also limited to BB store items.
 
Smart $ said:
Don't forget about 250.122(B) in doing so... :slaphead:
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Ah, that. I've had this argument before; changing the breaker is not increasing conductor size. 25A circuits arent listed in T250.122. EGC is at least 10ga cu. 250.122(B) does not say "or if MOCP is downsized, thus effectively increasing conductor size...".

Is 250.122(B) the same in the '14 as the '08? I use the 08 because VA resi is actually based on the 2012 IRC, which (I think) is mainly based on the 08 NEC, tho there are significant exceptions, usually toward the more relaxed side of things (AFCI breaker req's are basically 2002 NEC).
 
JFletcher said:
Ah, that. I've had this argument before; changing the breaker is not increasing conductor size. 25A circuits arent listed in T250.122. EGC is at least 10ga cu. 250.122(B) does not say "or if MOCP is downsized, thus effectively increasing conductor size...".
...
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Justifications abound... and yours is just one more in the pot!!!

250.122(B) has been essentially the same since 2002. A few words got changed to clarify the intent... but that's all.
 
JFletcher said:
A.

Is 250.122(B) the same in the '14 as the '08? I use the 08 because VA resi is actually based on the 2012 IRC, which (I think) is mainly based on the 08 NEC, tho there are significant exceptions, usually toward the more relaxed side of things (AFCI breaker req's are basically 2002 NEC).
Click to expand...



The 2012 IRC Electrical would be based the 2011 NEC.

(AFCI breaker req's are basically 2002 NEC)
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Try 2005 NEC
 
Smart $ said:
Justifications abound... and yours is just one more in the pot!!!

250.122(B) has been essentially the same since 2002. A few words got changed to clarify the intent... but that's all.
Click to expand...

I have to read the code as literally as I can. I am not changing the ungrounded conductor size; it is existing wiring. 250.122(B) is simply n/a here imo, tho I'm open to any convincing argument to the contrary. I am not changing the conductor size, regardless of what breaker I may (will) install.
 
JFletcher said:
Fuses would be if the discos werent trashed. Since I've got to replace the disconnects, I can save the customer money and have a simpler (one OCPD) system by doing unfused discos and the proper breaker.

If it were a FPE panel or something similarly obsolete/problematic, I'd probably just get new discos with 25A fuses and not touch the panel. By doing this on a weekend, I'm also limited to BB store items.
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I am certain that you have far more electrical experience than I do. So, please keep in mind that I am not challenging your knowledge.

I would like to discuss, however, the possibility of the circuit you are working on at some point in the future being required to carry a 40Amp load again. With my limited experience, if I were to look in a main panel and see a 25 amp breaker with 8AWG wire under the terminal screws, I would not automatically assume that I could replace the 25 amp breaker with a 40 amp breaker to bring the capacity of the circuit back up.

However, if I saw a 40 amp breaker with 8AWG in the main panel, and 8AWG coming into a disco with a 25 amp breaker or fuse, I would be quite comfortable returning the load on the circuit to 40 amp.

Please let me know if I am seeing this issue incorrectly.
 
David Goodman said:
I am certain that you have far more electrical experience than I do. So, please keep in mind that I am not challenging your knowledge.

I would like to discuss, however, the possibility of the circuit you are working on at some point in the future being required to carry a 40Amp load again. With my limited experience, if I were to look in a main panel and see a 25 amp breaker with 8AWG wire under the terminal screws, I would not automatically assume that I could replace the 25 amp breaker with a 40 amp breaker to bring the capacity of the circuit back up.

However, if I saw a 40 amp breaker with 8AWG in the main panel, and 8AWG coming into a disco with a 25 amp breaker or fuse, I would be quite comfortable returning the load on the circuit to 40 amp.

Please let me know if I am seeing this issue incorrectly.
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There is some validity to your thought there. But for an air conditioner you very well can run into 8 AWG and a 60 amp breaker as well.

You need to assess what is there before making such decisions when making such a change.

Most dwellings the length of run is not much factor to cause upsizing for voltage drop either, especially on a 240 volt circuit.
 
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